Such a method is disclosed in Patent Document 1 that produces an optical waveguide by forming a continuous high refractive index area on the basis of continuously scanning a glass material utilizing the increase in the refractive index in the vicinity of the focal point when femtosecond laser pulses are focused inside transparent glass. The method has a feature in that the number of processes is smaller than a method for producing an optical waveguide device using a prior art ion exchange method and flame deposition method. In addition, with this method, since a change in the refractive index is induced only in the vicinity of the focal point of the laser, it is possible to produce a waveguide device having an arbitrary structure inside glass.
However, a fact that, in an optical waveguide produced by the femtosecond laser pulses, the propagation loss is larger than a waveguide produced by using a prior art flame deposition method, etc., remained unsolved problem in practical applications. It is considered that the causes of the propagation loss herein are a scattering loss due to density fluctuations and an incomplete-structure loss due to fluctuations in the core-clad boundary. Therefore, a method is disclosed in Patent Document 2, which reduces a loss in a waveguide by using a glass material including a dopant to control the refractive index.    [Patent Document 1] Japanese Unexamined Patent Application Publication No. Hei-9-311237    [Patent Document 2] Japanese Unexamined Patent Application Publication No. 2002-311277